The electrical systems in residential, commercial and industrial applications usually include a panelboard for receiving electrical power from a utility source. The power is then routed through protection devices to designated branch circuits supplying one or more loads. These overcurrent devices are typically circuit interrupters such as circuit breakers and fuses which are designed to interrupt the electrical current if the limits of the conductors supplying the loads are surpassed.
Circuit breakers are a preferred type of circuit interrupter because a resetting mechanism allows for their reuse. Typically, circuit breakers interrupt an electric circuit due to a disconnect or trip condition such as a current overload or ground fault. The current overload condition results when a current exceeds the continuous rating of the breaker for a time interval determined by the current. A ground fault trip condition is created by an imbalance of currents flowing between a line conductor and a neutral conductor which could be caused by a leakage current or an arcing fault to ground.
Arcing faults are commonly defined as current through ionized gas between two ends of a broken conductor or at a faulty contact or connector, between two conductors supplying a load, or between a conductor and ground. However, arcing faults may not cause a conventional circuit breaker to trip. Arcing fault current levels may be reduced by branch or load impedance to a level below the trip curve settings of the circuit breaker. In addition, an arcing fault which does not contact a grounded conductor or person will not trip a ground fault protector.
There are many conditions that may cause an arcing fault. For example, corroded, worn or aged wiring, connectors, contacts or insulation, loose connections, wiring damaged by nails or staples through the insulation, and electrical stress caused by repeated overloading, lightning strikes, etc. These faults may damage the conductor insulation and/or cause the conductor to reach an unacceptable temperature.
National Electrical code requirements presently allow arc fault circuit interrupters (AFCI's) for outlets supplying bedroom branch circuits in all dwelling units to be of the “branch feeder” type. The branch feeder is capable of detecting parallel arc faults (i.e., arcing across the line) at or above 75 amperes. These devices are primarily designed to protect the branch wiring from the circuit breaker panel to the outlet and to some degree devices plugged into those outlets. Effective Jan. 1, 2008, the National Electrical code will require “combination” type AFCI's providing a higher degree of protection for outlets supplying bedroom branch circuits. Combination AFCI's are capable of providing all the protection afforded by a branch feeder AFCI, but are further capable of detecting series arc faults (i.e., an arc in series with either line conductor) as low as 5 amperes. Such a fault could occur, for example, in a lamp or appliance cord for a device plugged into an outlet.
It is known in the art of branch feeder arc fault detection to measure high frequency spectral components in the load current signature of an arcing load. If sufficient spectral content is present in certain frequency bands, this can be taken into account and used to detect the arc fault using a signal processing detection algorithm. One of the challenges in detecting series arc faults at the relatively lower amperage detection level standard specified for a combination type AFCI is correctly measuring the amplitude of high frequency components because these high frequency components are reduced, in comparison to branch feeder arc faults, at the low arcing current levels of a series arc fault. This challenge is exacerbated by the fact that inductive loads, such as an electric motor, may be present in series with the arc, and these loads tend to attenuate the amplitude of the high frequency signature. An additional problem is presented by the presence of personal computers and other electronic equipment which provide capacitive filtering of the power line. These filters effectively short out part of the high frequency signal when connected to the same circuit as the arc which needs to be detected.
Embodiments of the present invention address a need in the art for a combination type AFCI. More specifically, embodiments of the present invention address a need in the art for detecting the presence of high frequency current components at low level amperage series arcing currents.